This application claims the priority of German Application No. 197 47 670.8, filed on Oct. 29, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an emission control system for an internal-combustion engine having at least two nitrogen oxide adsorbers arranged in parallel for the alternating periodic adsorption and desorption of nitrogen oxides contained in the exhaust gas of the internal-combustion engine and devices for providing a desorption gas flow for the nitrogen oxide adsorbers running in each case in the desorption operation.
Nitrogen oxide adsorbers in emission control systems are known for adsorbing in the adsorption operation nitrogen oxides contained in the exhaust gas and thus removing them from the exhaust gas which can then be discharged to the outside largely without nitrogen oxides. From time to time, i.e., no later than when it reaches its nitrogen oxide storage capacity, the respective nitrogen oxide adsorber is run in the desorption operation in which the nitrogen oxides adsorbed therein are desorbed again. The exhaust gas flow which takes up the desorbed nitrogen oxides will then be treated appropriately, for example, and will be returned into an intake pipe of the internal-combustion engine in order to react the desorbed nitrogen oxides by the combustion in the internal-combustion engine to nitrogen and oxygen.
An emission control system is described in DE 43 19 294 C1. There, two parallel nitrogen oxide adsorbers are alternatingly run in the adsorption and in the desorption operation. For the desorption, fresh air is fed into the corresponding nitrogen oxide adsorber by way of an air pipe and this nitrogen oxide adsorber or the fed fresh air is heated externally while simultaneously the feeding of the exhaust gas flow remains blocked. In contrast, the feeding of air and the external heating of the respective adsorber do not take place in the adsorption operation in which the exhaust gas flow of the internal-combustion engine flows through the adsorber.
German Patent Application No. 196 28 796.0 describes an emission control system which has two adsorbers, which are arranged in parallel and are alternatingly run in the adsorption and in the desorption operation, and an oxidation catalyst arranged downstream of the adsorbers close to the engine, for the oxidation of NO contained in the exhaust gas to NO.sub.2. An exhaust pipe section situated upstream of the adsorbers is divided into a main flow pipe branch and into a partial flow pipe branch which is parallel to the main flow pipe branch and is shorter and/or more thermally insulated than the main flow pipe branch. The adsorber which, in each case, is operated in the adsorption mode is fed by the exhaust gas flow of the main flow pipe branch. and the respective other adsorber operated in the desorption mode is fed by the exhaust gas flow of the partial flow pipe branch. This measure takes into account the fact that a low temperature level promotes the adsorption operation and a higher temperature level promotes the desorption operation and nitrogen oxide adsorbers frequently adsorb NO.sub.2 much more easily than NO.
Another emission control system with two nitrogen oxide adsorbers arranged in parallel is disclosed in DE 196 26 837 A1. There, the desorption gas flow providing devices consist, for example, of two electrically heatable catalysts which are connected directly in front of the respective nitrogen oxide adsorber and in front of which, in the respective branch of the exhaust pipe system, a respective fuel injecting unit is connected; or the desorption gas flow providing devices consist of a burner which is assigned jointly to the nitrogen oxide adsorbers and is operated by a slightly lean diesel fuel/air mixture, and of a fuel injecting unit which is connected behind this burner. In the latter, the hot burner exhaust gas acts as a carrier gas for the injected fuel which is fed to the respective nitrogen oxide adsorber by way of a corresponding pipe branch. One or two lambda probes, which are arranged in parallel downstream of the nitrogen oxide adsorbers recognize the end of a respective desorption phase.